In recent years, internal combustion engines employing a premixed compression self-ignition combustion method, in which an air-fuel mixture is compressed to cause self-ignition thereof, have been drawing attention. The internal combustion engines employing the premixed compression self-ignition combustion method are known to exhibit better performances in points of improved fuel consumption, NOx reduction, and the like than that of the conventional internal combustion engines employing a diffusion combustion method or a flame propagation combustion method.
The conventional internal combustion engine employing the premixed compression self-ignition combustion method is disclosed in, for example, Patent Document 1. In the internal combustion engine described in Patent Document 1, as illustrated by a map indicating a relation between a load and a revolution number, the premixed compression self-ignition combustion is selected to be performed on a low-revolution/low-load side, and jump spark ignition combustion is selected to be performed on a high-revolution/high-load side. During the premixed compression self-ignition combustion, a sealing period in which both an intake valve and an exhaust valve are closed is set around an exhaust top dead center to perform internal EGR for causing burnt gas to remain in a combustion chamber, and the timings for opening/closing the intake valve and the exhaust valve are adjusted according to changes in the external environment to ensure stable self-ignition combustion. For example, the sealing period is prolonged as the temperature of intake air drops so that the temperature in the combustion chamber is prevented from dropping in the neighborhood of the exhaust top dead center. It has also been proposed to inject fuel into the combustion chamber during the sealing period to generate a modified substance exhibiting high ignitability.
Patent Document 1: JP 2002-129991 A